"an option to change the encoder to use"
Scroll down in the render settings, you can configure the exact program and arguments called used by the ReplayMod in the Command Line Settings.
"I can’t download it for many reasons"
Unless it's illegal in the country you're in, I can't think of a single one.
"one of the, is that I have MKVToolNix"
I see no connection there.
"and it’s not my computer"
But you can use a mod? There's no significant technical difference between ffmpeg and the ReplayMod. ffmpeg has portable versions as well.